This invention relates to a technique for encoding digital signals to have minimal DC component and, more particularly, to a so-called "block-coding" technique wherein an m-bit code signal is used to represent a n-bit digital word, with the m-bit code signal exhibiting zero disparity for most of the n-bit digital words represented thereby, and also exhibiting minimal run-length for those digital words having a greater probability of occurrence.
When digital information is transmitted or recorded on, for example, a magnetic medium, the DC levels, or components, which represent each binary "1" and "0" of the digital signals often is so distorted that the original digital signal cannot be reproduced with any degree of accuracy. For example, in magnetic recording of digital information, such DC components are blocked by the usual recording amplifiers and recording heads, as well as the playback amplifiers and playback heads. For the application of digital recording techniques in the field of video information, such as in a digital video tape recorder (VTR), such DC components are additionally blocked by the particular circuitry and electrical elements which comprise the digital VTR. Since distortion of the digital signals which ultimately are reproduced from the magnetic medium results in serious degradation of the television picture which is produced therefrom, it is desirable to use signal modulation techniques so as to record a suitably modulated signal that exhibits good noise immunity and, moreover, has a substantially zero DC component. It is throught that if the DC component of the digital signal, that is, the overall or average DC value for all of the bits constituting that signal is equal to zero, then the fact that the DC component is blocked because of the inherent characteristics of the recording and reproducing apparatus will not distort the digital signal which can be recovered. One example of an encoding technique which has been proposed to minimize this DC component of the digital signal is described in copending U.S. application Ser. No. 192,358, filed Sept. 30, 1980.
The encoding of an n-bit digital bit signal to an m-bit code word is referred to generally herein as "block encoding". If a binary "1" is represented by a DC level of, for example, +1, and if a binary "0" is represented by a level of -1, then a binary "1" followed by a binary "0" [10] will exhibit a zero DC component. A combination of bits, such as [100] exhibits a DC component of -1. A combination of bits [1001] exhibits a DC component of zero. Thus, a plural-bit word may be thought of as having a DC component that is equal to the difference between the number of binary "1"s and "0"s contained in that word. This DC component is referred to herein as the "disparity" of that word. If the disparity of a digital word is positive, then the number of binary "1"s exceeds the number of binary "0"s in that word. Conversely, if the disparity of a digital word is negative, then the number of binary "1"is is less than the number of binary "0"s. For a digital word formed of an even number of bits, the disparity thereof is zero if the number of binary "1"s is equal to the number of binary "0"s. Of course, an even-numbered bit word may exhibit positive or negative disparity, depending upon whether the number of binary "1"s exceeds or is less than the number of binary "0"s.
It is thought that, if every n-bit digital word can be converted to an m-bit code word wherein each such m-bit code word exhibits zero disparity, then the overall DC component of those code words which are recorded will be equal to zero. Hence, distortion and degradation in the reproduced digital signal can be minimized. It is seen that, if n=4, then a total of sixteen different digital words can be constructed of these four bits. Now, if m=6, then a total of twenty 6-bit zero disparity code words are available. Accordingly, each 4-bit word can be represented by a single, corresponding 6-bit zero disparity code word. However, in many applications, such as in digital video recording, the original digital signal is formed of eight bits. With particular reference to a digital VTR, each video signal level is sampled and quantized into one of 256 different quantizing levels. Each quantizing level thus is represented by a respective one of 256 different 8-bit signals. If each 8-bit signal is to be represented by one 10-bit code word, then, of the 1,024 different 10-bit code words which are available, only 252 exhibit zero disparity. Consequently, if each 8-bit digital signal is to be represented by a zero disparity code word, then each code word must be formed of twelve bits. But, since only eight bits are needed to represent useful information, it is seen that, in this block encoding scheme, four redundant bits must be used. This adds to the bit density of the signals which must be recorded, and is undesirable.
It is one advantageous feature of the present invention to minimize the number of redundant bits which are used in the m-bit code word, even though this provides a smaller number of zero disparity code bits than there are n-bit digital signals. To represent the "surplus" n-bit digital signals, m-bit code words having non-zero disparity are used, but such code words are assigned to represent n-bit digital signals having a relatively low probability of occurrence. Also, in the event that one of these surplus n-bit digital signal occurs, it will be represented by a non-zero disparity m-bit code word of one polarity (either positive--representing a greater number of binary "1"s than "0"s, or negative--representing a greater number of binary "0"s than "1"s), and when another surplus n-bit digital signal occurs, it will be represented by a non-zero disparity m-bit code word of opposite polarity. Thus, the overall DC component nevertheless will be minimized.
If it is possible to generate code words having only zero disparity, it is appreciated that some of these code words will exhibit substantial "run lengths". The run length of a digital signal is the number of binary "1"s or "0"s that are repeated in succession. For example, the 10-bit code word [1010101010] exhibits minimal run length; the 10-bit code word [1110000101] exhibits substantially greater run length; and the code word [1111100000] exhibits maximum run length. When a digital signal having a large run length is recorded magnetically, the phenomenon of "peak shift" may occur when that signal is reproduced. Such peak shift results in the reproduction of distorted data.
It is another advantageous feature of the present invention to assign code words having minimal run length to those digital signals having a higher probability of occurring, and to assign those code words having progressively larger run lengths to those digital signals having a progressively smaller probability of occurring. Analogous to this feature, code words having minimal run length may be assigned to those digital signals wherein distortion, due to peak shift or the like, results in greater interference. For example, if this invention is used in a digital VTR, and if the analog video signal is quantized over a range of 256 quantizing levels, then code words having minimal run length and zero disparity are assigned to those quantizing levels which are in substantially the center portion of this quantizing range.